Acid batteries (accumulators) are used in the most widely varying areas, for example in vehicles as starter batteries or as a sole energy storage and delivery means. Usually the batteries comprise one or more cell vessels with plate blocks which are fitted therein, comprising positive and negative plates. The plates of each plate set have projections which protrude from the plates, referred to as lugs, and which are electrically connected together by way of connecting straps. The connecting straps are in turn connected to the poles which are usually passed to the exterior through openings (pole ductings) in the cover of the cell vessel for electrical connection of the battery. The plates are surrounded by electrolyte in the cell vessel, usually an acid electrolyte based on sulphuric acid. The electrolyte can be in the form of a liquid electrolyte or also fixed in a get as so-called gel electrolyte. Silicic acid is usually employed as the gel-forming agent.
By virtue of the generation of gas in the electrolytic processes in the battery and in order to compensate for fluctuations in pressure due to the generation of heat, the cell vessels of acid batteries may not be gas-tightly or pressure-tightly closed but have one or more vent valves. The vent valves provide for pressure equalisation between the interior of the cell vessels and the ambient atmosphere in the event of a high pressure in the cell with respect to the ambient atmosphere. As the acid electrolyte which is usually based on sulphuric acid is very corrosive, the vent valves should be so designed that, when gas issues from the cell vessel, no acid is entrained out of the cell vessel with the gas, in the form of liquid or in the form of acid mist.
Acid batteries are transported on land, across sea and by rail and also by air freight. In accordance with those transport regulations a battery is classified as being non-spillable if it stands up inter alia to a) a vibration test and b) a reduced-pressure test, in accordance with the following test provisions.
a) Vibration Test
The battery is secured on the platform on a vibrator device and a simple harmonic movement involving an amplitude of 0.8 mm (1.6 mm maximum total deflection) is applied. The vibration frequency is varied at a rate of 1 Hz/min from 10 Hz to 55 Hz and back to 10 Hz. The entire frequency range from the lowest frequency to the highest frequency and back again is covered in 95±5 minutes for each individual securing position (vibration direction) for the battery. The battery has to be tested in three mutually perpendicularly positions, in which respect at least one upside-down position must be included, in which the filling openings and vent valves of the battery face down.
b) Reduced-Pressure Test
After the vibration test the same battery is stored for 6 hours at 24±4° C., being exposed to a pressure difference of at least 88 kPa. The battery is tested once again in three mutually perpendicular positions for at least 6 hours in each position, wherein in this test also at least one upside-down position must be included, in regard to the filling opening and the vent valves.
As the filling openings of acid batteries can be well sealed off, the greatest danger of battery acid escaping in the vibration test and in the reduced-pressure test occurs in the upside-down positions. Most conventional vent valves in accordance with the state of the art do not satisfy those severe demands.
EP 0 638 944 A1 describes a closure plug for a lead accumulator, which is intended to prevent acid mists from escaping from the cell and also to exclude the damaging influence of the acid on the function of a safety valve provided in the closure plug. The closure plug includes a microporous frit and a pressure relief valve in a housing which can be fixed in sealed relationship with respect to the cell cover in the electrolyte filling opening. The microporous frit is disposed upstream of the pressure relief valve in the gas discharge direction. The microporous frit is intended to catch fine droplets of acid aerosol so that the gas flow reaching the pressure valve is freed of the liquid and cannot cause any corrosion damage to the delicate metal parts such as compression springs and the like. It has been found however that this arrangement cannot adequately prevent the escape of acid mists under severe conditions like the above-described tests.
DE 39 34 687 C1 discloses a closed, maintenance-free accumulator with a fixed electrolyte, which can be used in an upright and a lying position. A filling opening with an excess-pressure valve is provided in the cover of the accumulator housing. Provided towards the inside of the housing, at the ducting opening, is a projection from which a pipe extends parallel to the cover wall and opens into the gas space in the upper region of the accumulator. The path of gas issuing from the accumulator therefore leads through the pipe to the projection in the housing cover and then outwardly through the excess-pressure valve. In the lying position of the accumulator, the pipe must face upwardly in a vertical direction with its mouth opening so that small amounts of liquid which issue from the fixed electrolyte cannot pass into the upwardly disposed mouth of the pipe and thus out of the accumulator housing.
Further vent valves and closure plugs for accumulators are described in the following publications: EP 0 920 063 A1, EP 0 756 338 A1, FR 2 576 149, EP 0 588 823 A1, GB 567 824, EP 1 001 905 A1, EP 0 875 949 B1, EP 0 588 823 A1, U.S. Pat. No. 4,352,364, FR 2 674 376, FR 2 551 172, EP 0 554 535 A1, EP 0 504 573 and EP 0 222 447 A.
The known closure systems and vent valves either suffer from the disadvantage that they do not guarantee adequate protection from the escape of battery acid under severe conditions or they are of a technically very expensive and complicated structure, or both.